Origin: 1250–1300; Middle English posterne < Old French, variant of posterle < Late Latin posterula, diminutive of postera back door, noun use of feminine of posterus coming behind. See posterior, -ule
late 13c., "back door, private door," from O.Fr. posterne, earlier posterle, from L.L. posterula "small back door or gate," dim. of L. posterus "that is behind, coming after, subsequent," from post "after."